Continuous casting, in the steel-making industry, is the key process whereby molten steel is solidified into a semifinished product such as a billet, bloom, or slab for subsequent rolling in the hot strip mill or the finishing mill. This process is achieved through a well-designed casting machine, known as a continuous caster, or concaster.
FIG. 1 shows a schematic diagram of a continuous caster according to the prior art, which comprises the following key sections: a ladle turret 20, a ladle 22, a tundish 24 with a stopper-rod 26, a submerged entry nozzle (SEN) 28, a water-cooled copper mold 30, a roller containment section with additional cooling chambers 32, a straightener withdrawal unit 34 and a torch severing equipment 36.
Molten steel from an electric or basic oxygen furnace is tapped into a ladle and shipped to the continuous caster. The ladle is placed into the casting position above the tundish 24 by the turret 20. The steel is poured into the tundish 24, and then into the water-cooled copper mold 30 through the SEN 28, which is used to regulate the steel flow rate and provide precise control of the steel level 38 in the mold. As the molten steel moves down the mold 30 at a controlled rate, the outer shell of the steel becomes solidified to produce a steel strand 40. Upon exiting the mold 30, the strand 40 enters a roller containment section and cooling chamber in which the solidifying strand is sprayed with water to promote solidification. Once the strand is fully solidified and has passed through the straightener withdrawal unit 34, it is cut to the required length in the severing unit 36.
The main operational issues in continuous casting processes relate to achieving a stable operation following start-up, and then maintaining stability. A proper start-up operation is very crucial to successfully achieving this goal, which involves appropriate use of a dummy bar, the correct starting lubricant and the applicable sequence of ramping up to the casting speed during the start-up operation.
To start a cast, the mold bottom is sealed by a steel dummy bar, which prevents molten steel from flowing out of the mold. The steel poured into the mold is partially solidified, producing a steel strand with a solid outer shell 42 and a liquid core 44. Once the steel shell has a sufficient thickness, the straightener withdrawal unit withdraws the partially solidified strand out of the mold along with the dummy bar. Molten steel continues to pour into the mold to replenish the withdrawn steel at an equal rate. When the dummy bar head, which is now attached to the solidified strand being cast, reaches a certain position in the withdrawal unit, it is mechanically disconnected and removed.
A well-known problem associated with the continuous caster, is that molten steel is prone to tear in the strand shell and cause a breakout such that molten steel pours out beneath the mold. A breakout may occur either during start-up operation, known as a start cast breakout, or during the following run-time operation, known as a run-time cast breakout. For a typical, fully operational continuous caster, approximately 25% of total breakouts occur during the start-up operation. These breakouts are of major concern in the steel-making industry, because they diminish the reliability and efficiency of the production process, create substantial costs due to production delays and destruction of equipment, and many times, pose significant safety risks to plant operators. Therefore, the ability to prevent breakouts from happening utilizing engineering expertise and analytical methods can provide excellent benefits to the continuous casting process.
Although there have already been some methods and systems developed to detect and/or predict the run-time cast breakouts in the prior art, the start cast breakout and its prevention has received very little attention in both academia and industry. It is important, then, to be able to predict start cast breakouts with sufficient lead-time such that they can be prevented by taking appropriate control actions. One example of these control actions is to change the ramping profile of the casting speed in order to slow down the casting process and provide more time for steel solidification in the mold.
According to the prior art in the area of detecting and/or predicting breakouts in continuous casting processes, there exist two different types of methods. One is the pattern-matching method, for example, the well-known sticker detection method, which develops comprehensive rules to characterize the patterns in the mold temperatures prior to the incidence of a breakout based on past casting operation experiences. If such patterns have been recognized in the current casting operation, then there is a high likelihood that a breakout will occur. The relevant systems based on this type of method are described by Yamamoto et al in U.S. Pat. No. 4,55,099, Blazek et al in U.S. Pat. No. 5,020,585, Nakamura et al in U.S. Pat. No. 5,548,520, and by Adamy in U.S. Pat. No. 5,904,202. The other method is multivariable statistical method described by Vaculik et al in U.S. Pat. No. 6,564,119 where a principal component analysis (PCA) model is built using an extended set of process measurements, beyond the standard mold temperatures, to model the normal operation of casting processes; certain statistics are then calculated by the model to detect exceptions to normal operation in the current casting operation and predict potential breakouts. Both of these methods, however, are focused on detecting and/or predicting the run-time cast breakouts, and will experience some difficulties when they are applied to the start-up operation.
The applicant is also aware of prior art in the use of multivariable statistical technology for batch process monitoring and fault diagnosis in other fields. Examples of methods and industrial applications of monitoring a batch process using multivariate statistical technology are described by MacGregor and his co-workers in AIChE Journal, volume 40, 1994, Journal of Process Control, volume 5, 1995, etc. There is no application of such multivariable statistical technology to continuous caster start-up operations described in the patent literature.
To summarize, methods and online systems for monitoring continuous caster start-up operations and predicting start cast breakouts using multivariable statistical technology have not been addressed to date.